Metrology Method and Apparatus, Computer Program and Lithographic System

ABSTRACT

Disclosed is a metrology apparatus for measuring a parameter of a lithographic process, and associated computer program and method. The metrology apparatus comprises an optical system for measuring a target on a substrate by illuminating the target with measurement radiation and detecting the measurement radiation scattered by the target; and an array of lenses. Each lens of the array is operable to focus the scattered measurement radiation onto a sensor, said array of lenses thereby forming an image on the sensor which comprises a plurality of sub-images, each sub-image being formed by a corresponding lens of the array of lenses. The resulting plenoptic image comprises image plane information from the sub-images, wavefront distortion information (from the relative positions of the sub-images) and pupil information from the relative intensities of the sub-images.

BACKGROUND

Field of the Invention

The present invention relates to methods and apparatus for metrologyusable, for example, in the manufacture of devices by lithographictechniques and to methods of manufacturing devices using lithographictechniques.

Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. In lithographic processes, itis desirable frequently to make measurements of the structures created,e.g., for process control and verification. Various tools for makingsuch measurements are known, including scanning electron microscopes,which are often used to measure critical dimension (CD), and specializedtools to measure overlay, a measure of the accuracy of alignment of twolayers in a device. Overlay may be described in terms of the degree ofmisalignment between the two layers, for example reference to a measuredoverlay of 1 nm may describe a situation where two layers are misalignedby 1 nm.

Recently, various forms of scatterometers have been developed for use inthe lithographic field. These devices direct a beam of radiation onto atarget and measure one or more properties of the scatteredradiation—e.g., intensity at a single angle of reflection as a functionof wavelength; intensity at one or more wavelengths as a function ofreflected angle; or polarization as a function of reflected angle—toobtain a “spectrum” from which a property of interest of the target canbe determined. Determination of the property of interest may beperformed by various techniques: e.g., reconstruction of the target byiterative approaches such as rigorous coupled wave analysis or finiteelement methods; library searches; and principal component analysis.

The targets used by conventional scatterometers are relatively large,e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spotthat is smaller than the grating (i.e., the grating is underfilled).This simplifies mathematical reconstruction of the target as it can beregarded as infinite. However, in order to reduce the size of thetargets, e.g., to 10 μm by 10 μm or less, e.g., so they can bepositioned in amongst product features, rather than in the scribe lane,metrology has been proposed in which the grating is made smaller thanthe measurement spot (i.e., the grating is overfilled). Typically suchtargets are measured using dark field scatterometry in which the zerothorder of diffraction (corresponding to a specular reflection) isblocked, and only higher orders processed. Examples of dark fieldmetrology can be found in international patent applications WO2009/078708 and WO 2009/106279 which documents are hereby incorporatedby reference in their entirety. Further developments of the techniquehave been described in patent publications US20110027704A,US20110043791A and US20120242970A. The contents of all theseapplications are also incorporated herein by reference.Diffraction-based overlay using dark-field detection of the diffractionorders enables overlay measurements on smaller targets. These targetscan be smaller than the illumination spot and may be surrounded byproduct structures on a wafer. Targets can comprise multiple gratingswhich can be measured in one image.

In the known metrology technique, overlay measurement results areobtained by measuring the target twice under certain conditions, whileeither rotating the target or changing the illumination mode or imagingmode to obtain separately the −1^(st) and the +1^(st) diffraction orderintensities. The intensity asymmetry, a comparison of these diffractionorder intensities, for a given target provides a measurement of targetasymmetry, that is asymmetry in the target. This asymmetry in the targetcan be used as an indicator of overlay error (undesired misalignment oftwo layers).

Although the known dark-field image-based overlay measurements are fastand computationally very simple (once calibrated), they rely on anassumption that overlay (i.e., overlay error and deliberate bias) is theonly cause of target asymmetry in the target. Any other asymmetry in thetarget, such as structural asymmetry of features within one or both ofthe overlaid gratings, also causes an intensity asymmetry in the 1^(st)(or other higher) orders. This intensity asymmetry attributable tostructural asymmetry, and which is not related to overlay, clearlyperturbs the overlay measurement, giving an inaccurate overlaymeasurement. Asymmetry in the lowermost or bottom grating of a target isa common form of structural asymmetry. It may originate for example inwafer processing steps such as chemical-mechanical polishing (CMP),performed after the bottom grating was originally formed.

Therefore, it is desired to distinguish the contributions to targetasymmetry that are caused by overlay error and other effects in a moredirect and faster way. It is also desired to simplify the apparatusrequired for focus and/or aberration measurement within a metrologysystem. It is further desired to be able to perform critical dimensionmeasurements (and other reconstruction techniques) on small overfilledtargets.

SUMMARY OF THE INVENTION

The invention in a first aspect provides a metrology apparatus formeasuring a parameter of a lithographic process, the metrology apparatuscomprising:

an optical system for measuring a target on a substrate by illuminatingthe target with measurement radiation and detecting the measurementradiation scattered by the target; and an array of lenses, each lensbeing operable to focus the scattered measurement radiation onto asensor, said array of lenses thereby forming an image on the sensor suchthat said image comprises a plurality of sub-images, each sub-imagebeing formed by a corresponding lens of said array of lenses.

The invention in a further aspect provides a method of measuring aparameter of a lithographic process, comprising measuring a target on asubstrate by illuminating the target with measurement radiation anddetecting the measurement radiation scattered by the target; forming animage of the target, said image comprising a plurality of sub-images,each sub-image being formed by a corresponding lens of an array oflenses; and measuring said parameter of a lithographic process from saidimage.

The invention further provides a computer program comprising processorreadable instructions which, when run on suitable processor controlledapparatus, cause the processor controlled apparatus to perform themethod of the first aspect or the second aspect, and a computer programcarrier comprising such a computer program. The processor controlledapparatus may comprise the metrology apparatus of the third aspect orthe lithographic system of the fourth aspect.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a lithographic cell or cluster according to an embodimentof the invention;

FIGS. 3A-3D comprise 3A a schematic diagram of a dark fieldscatterometer for use in measuring targets using a first pair ofillumination apertures, 3B a detail of diffraction spectrum of a targetgrating for a given direction of illumination 3C a second pair ofillumination apertures providing further illumination modes in using thescatterometer for diffraction based overlay measurements and 3D a thirdpair of illumination apertures combining the first and second pair ofapertures;

FIG. 4 depicts a known form of multiple grating target and an outline ofa measurement spot on a substrate;

FIG. 5 depicts an image of the target of FIG. 4 obtained in thescatterometer of FIG. 3;

FIG. 6 is a flowchart showing the steps of an overlay measurement methodusing the scatterometer of FIG. 3 and adaptable to form embodiments ofthe present invention;

FIGS. 7A to 7C show schematic cross-sections of overlay gratings havingdifferent overlay values in the region of zero;

FIG. 7D is a schematic cross-section of an overlay grating havingstructural asymmetry in a bottom grating due to processing effects;

FIG. 8 illustrates known principles of overlay measurement in an idealtarget, not subject to structural asymmetry;

FIG. 9 illustrates a principle of overlay measurement in a non-idealtarget, with correction of structural asymmetry as disclosed inembodiments of the invention;

FIG. 10 is a schematic diagram of a dark field scatterometer accordingto an embodiment of the invention; and

FIG. 11 is a flowchart of the steps of a method according to anexemplary embodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination optical system (illuminator) IL configured tocondition a radiation beam B (e.g., UV radiation or DUV radiation), apatterning device support or support structure (e.g., a mask table) MTconstructed to support a patterning device (e.g., a mask) MA andconnected to a first positioner PM configured to accurately position thepatterning device in accordance with certain parameters; a substratetable (e.g., a wafer table) WT constructed to hold a substrate (e.g., aresist coated wafer) W and connected to a second positioner PWconfigured to accurately position the substrate in accordance withcertain parameters; and a projection optical system (e.g., a refractiveprojection lens system) PS configured to project a pattern imparted tothe radiation beam B by patterning device MA onto a target portion C(e.g., including one or more dies) of the substrate W.

The illumination optical system may include various types of opticalcomponents, such as refractive, reflective, magnetic, electromagnetic,electrostatic or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection optical system PS, which focuses the beam onto a targetportion C of the substrate W, thereby projecting an image of the patternon the target portion C. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers is described further below.

Lithographic apparatus LA in this example is of a so-called dual stagetype which has two substrate tables WTa, WTb and two stations—anexposure station and a measurement station—between which the substratetables can be exchanged. While one substrate on one substrate table isbeing exposed at the exposure station, another substrate can be loadedonto the other substrate table at the measurement station and variouspreparatory steps carried out. The preparatory steps may include mappingthe surface control of the substrate using a level sensor LS andmeasuring the position of alignment markers on the substrate using analignment sensor AS. This enables a substantial increase in thethroughput of the apparatus.

The depicted apparatus can be used in a variety of modes, including forexample a step mode or a scan mode. The construction and operation oflithographic apparatus is well known to those skilled in the art andneed not be described further for an understanding of the presentinvention.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic system, referred to as a lithographic cell LC or alithocell or cluster. The lithographic cell LC may also includeapparatus to perform pre- and post-exposure processes on a substrate.Conventionally these include spin coaters SC to deposit resist layers,developers DE to develop exposed resist, chill plates CH and bake platesBK. A substrate handler, or robot, RO picks up substrates frominput/output ports I/O1, I/O2, moves them between the different processapparatus and delivers then to the loading bay LB of the lithographicapparatus. These devices, which are often collectively referred to asthe track, are under the control of a track control unit TCU which isitself controlled by the supervisory control system SCS, which alsocontrols the lithographic apparatus via lithography control unit LACU.Thus, the different apparatus can be operated to maximize throughput andprocessing efficiency.

A metrology apparatus is shown in FIG. 3(a). A target T and diffractedrays of measurement radiation used to illuminate the target areillustrated in more detail in FIG. 3(b). The metrology apparatusillustrated is of a type known as a dark field metrology apparatus. Themetrology apparatus may be a stand-alone device or incorporated ineither the lithographic apparatus LA, e.g., at the measurement station,or the lithographic cell LC. An optical axis, which has several branchesthroughout the apparatus, is represented by a dotted line O. In thisapparatus, light emitted by source 11 (e.g., a xenon lamp) is directedonto substrate W via a beam splitter 15 by an optical system comprisinglenses 12, 14 and objective lens 16. These lenses are arranged in adouble sequence of a 4F arrangement. A different lens arrangement can beused, provided that it still provides a substrate image onto a detector,and simultaneously allows for access of an intermediate pupil-plane forspatial-frequency filtering. Therefore, the angular range at which theradiation is incident on the substrate can be selected by defining aspatial intensity distribution in a plane that presents the spatialspectrum of the substrate plane, here referred to as a (conjugate) pupilplane. In particular, this can be done by inserting an aperture plate 13of suitable form between lenses 12 and 14, in a plane which is aback-projected image of the objective lens pupil plane. In the exampleillustrated, aperture plate 13 has different forms, labeled 13N and 13S,allowing different illumination modes to be selected. The illuminationsystem in the present examples forms an off-axis illumination mode. Inthe first illumination mode, aperture plate 13N provides off-axis from adirection designated, for the sake of description only, as ‘north’. In asecond illumination mode, aperture plate 13S is used to provide similarillumination, but from an opposite direction, labeled ‘south’. Othermodes of illumination are possible by using different apertures. Therest of the pupil plane is desirably dark as any unnecessary lightoutside the desired illumination mode will interfere with the desiredmeasurement signals.

As shown in FIG. 3(b), target T is placed with substrate W normal to theoptical axis O of objective lens 16. The substrate W may be supported bya support (not shown). A ray of measurement radiation I impinging ontarget T from an angle off the axis O gives rise to a zeroth order ray(solid line O) and two first order rays (dot-chain line +1 and doubledot-chain line −1). It should be remembered that with an overfilledsmall target, these rays are just one of many parallel rays covering thearea of the substrate including metrology target T and other features.Since the aperture in plate 13 has a finite width (necessary to admit auseful quantity of light, the incident rays I will in fact occupy arange of angles, and the diffracted rays 0 and +1/−1 will be spread outsomewhat. According to the point spread function of a small target, eachorder +1 and −1 will be further spread over a range of angles, not asingle ideal ray as shown. Note that the grating pitches of the targetsand the illumination angles can be designed or adjusted so that thefirst order rays entering the objective lens are closely aligned withthe central optical axis. The rays illustrated in FIGS. 3(a) and 3(b)are shown somewhat off axis, purely to enable them to be more easilydistinguished in the diagram.

At least the 0 and +1 orders diffracted by the target T on substrate Ware collected by objective lens 16 and directed back through beamsplitter 15. Returning to FIG. 3(a), both the first and secondillumination modes are illustrated, by designating diametricallyopposite apertures labeled as north (N) and south (S). When the incidentray I of measurement radiation is from the north side of the opticalaxis, that is when the first illumination mode is applied using apertureplate 13N, the +1 diffracted rays, which are labeled +1(N), enter theobjective lens 16. In contrast, when the second illumination mode isapplied using aperture plate 13S the −1 diffracted rays (labeled −1(S))are the ones which enter the lens 16.

A second beam splitter 17 divides the diffracted beams into twomeasurement branches. In a first measurement branch, optical system 18forms a diffraction spectrum (pupil plane image) of the target on firstsensor 19 (e.g. a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam. However, this requirement of a separate branch forfocusing is undesirable, increasing cost and complexity. One of the aimsof the disclosure removes the need for a separate focusing branch. Thepupil plane image can also be used for many measurement purposes such asreconstruction.

In the second measurement branch, optical system 20, 22 forms an imageof the target T on sensor 23 (e.g. a CCD or CMOS sensor). In the secondmeasurement branch, an aperture stop 21 is provided in a plane that isconjugate to the pupil-plane. Aperture stop 21 functions to block thezeroth order diffracted beam so that the image of the target formed onsensor 23 is formed only from the −1 or +1 first order beam. The imagescaptured by sensors 19 and 23 are output to processor PU which processesthe image, the function of which will depend on the particular type ofmeasurements being performed. Note that the term ‘image’ is used here ina broad sense. An image of the grating lines as such will not be formed,if only one of the −1 and +1 orders is present.

The particular forms of aperture plate 13 and field stop 21 shown inFIG. 3 are purely examples. In another embodiment of the invention,on-axis illumination of the targets is used and an aperture stop with anoff-axis aperture is used to pass substantially only one first order ofdiffracted light to the sensor. In yet other embodiments, 2^(nd), 3^(rd)and higher order beams (not shown in FIG. 3) can be used inmeasurements, instead of or in addition to the first order beams.

In order to make the measurement radiation adaptable to these differenttypes of measurement, the aperture plate 13 may comprise a number ofaperture patterns formed around a disc, which rotates to bring a desiredpattern into place. Note that aperture plate 13N or 13S can only be usedto measure gratings oriented in one direction (X or Y depending on theset-up). For measurement of an orthogonal grating, rotation of thetarget through 90° and 270° might be implemented. Different apertureplates are shown in FIGS. 3(c) and (d). The use of these, and numerousother variations and applications of the apparatus are described inprior published applications, mentioned above.

FIG. 4 depicts a target or composite target formed on a substrateaccording to known practice. The target in this example comprises foursub-targets (e.g., gratings) 32 to 35 positioned closely together sothat they will all be within a measurement spot 31 formed by themetrology radiation illumination beam of the metrology apparatus. Thefour targets thus are all simultaneously illuminated and simultaneouslyimaged on sensors 19 and 23. In an example dedicated to measurement ofoverlay, gratings 32 to 35 are themselves composite gratings formed byoverlying gratings that are patterned in different layers of thesemi-conductor device formed on substrate W. Gratings 32 to 35 may havedifferently biased overlay offsets in order to facilitate measurement ofoverlay between the layers in which the different parts of the compositegratings are formed. The meaning of overlay bias will be explained belowwith reference to FIG. 7. Gratings 32 to 35 may also differ in theirorientation, as shown, so as to diffract incoming radiation in X and Ydirections. In one example, gratings 32 and 34 are X-direction gratingswith biases of the +d, −d, respectively. Gratings 33 and 35 areY-direction gratings with offsets +d and −d respectively. Separateimages of these gratings can be identified in the image captured bysensor 23. This is only one example of a target. A target may comprisemore or fewer than 4 gratings, or only a single grating.

FIG. 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the target of FIG. 4 in the apparatus of FIG. 3,using the aperture plates 13NW or 13SE from FIG. 3(d). While the pupilplane image sensor 19 cannot resolve the different individual gratings32 to 35, the image sensor 23 can do so. The dark rectangle representsthe field of the image on the sensor, within which the illuminated spot31 on the substrate is imaged into a corresponding circular area 41.Within this, rectangular areas 42-45 represent the images of the smalltarget gratings 32 to 35. If the targets are located in product areas,product features may also be visible in the periphery of this imagefield. Image processor and controller PU processes these images usingpattern recognition to identify the separate images 42 to 45 of gratings32 to 35. In this way, the images do not have to be aligned veryprecisely at a specific location within the sensor frame, which greatlyimproves throughput of the measuring apparatus as a whole.

Once the separate images of the targets have been identified, theintensities of those individual images can be measured, e.g., byaveraging or summing selected pixel intensity values within theidentified areas. Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan important example of such a parameter.

One downside of such images is that no angle resolved information can beobtained from the image, only an averaged (or summed) intensity value.This means, for example, that it is not possible to measure overlay as afunction of angle. Only a single overlay value can be obtained, fromwhich it is not possible to distinguish actual overlay from intensityasymmetry resultant from structural asymmetry of the target.

FIG. 6 illustrates how, using for example the method described inapplication WO 2011/012624, overlay error (i.e., undesired andunintentional overlay misalignment) between the two layers containingthe component targets 32 to 35 is measured. Such a method may bereferred to a micro diffraction based overlay (μDBO). This measurementis done through target asymmetry, as revealed by comparing theirintensities in the +1 order and −1 order dark field images (theintensities of other corresponding higher orders can be compared, e.g.+2 and −2 orders) to obtain a measure of the intensity asymmetry. Atstep S1, the substrate, for example a semiconductor wafer, is processedthrough a lithographic apparatus, such as the lithographic cell of FIG.2, one or more times, to create a target including the gratings 32-35.At S2, using the metrology apparatus of FIG. 3 or FIG. 10, an image ofthe targets 32 to 35 is obtained using only one of the first orderdiffracted beams (say −1). At step S3, whether by changing theillumination mode, or changing the imaging mode, or by rotatingsubstrate W by 180° in the field of view of the metrology apparatus, asecond image of the targets using the other first order diffracted beam(+1) can be obtained. Consequently the +1 diffracted radiation iscaptured in the second image. By using prisms to separate the orders,separated images of the target formed by both +1 and −1 diffracted beams(and possibly also the zeroth order) can be obtained in a singlecapture.

Note that, by including only half of the first order diffractedradiation in each image, the ‘images’ referred to here are notconventional dark field microscopy images. The individual target linesof the targets will not be resolved. Each target will be representedsimply by an area of a certain intensity level. In step S4, a region ofinterest (ROI) is identified within the image of each component target,from which intensity levels will be measured.

Having identified the ROI for each individual target and measured itsintensity, the asymmetry of the target, and hence overlay error, canthen be determined. This is done (e.g., by the processor PU) in step S5comparing the intensity values obtained for +1 and −1 orders for eachtarget 32-35 to identify their intensity asymmetry, e.g., any differencein their intensity. The term “difference” is not intended to refer onlyto subtraction. Differences may be calculated in ratio form. In step S6the measured intensity asymmetries for a number of targets are used,together with knowledge of any known imposed overlay biases of thosetargets, to calculate one or more performance parameters of thelithographic process in the vicinity of the target T. In theapplications described herein, measurements using two or more differentmeasurement recipes will be included. A performance parameter of greatinterest is overlay. As will be described later, the novel methods alsoallow other parameters of performance of the lithographic process to becalculated. These can be fed back for improvement of the lithographicprocess, and/or used to improve the measurement and calculation processof FIG. 6 itself.

In the prior applications, mentioned above, various techniques aredisclosed for improving the quality of overlay measurements using thebasic method mentioned above. These techniques will not be explainedhere in further detail. They may be used in combination with thetechniques newly disclosed in the present application, which will now bedescribed.

FIG. 7 shows schematic cross sections of targets (overlay gratings),with different biases. These can be used as the target T on substrate W,as seen in FIGS. 3 and 4. Gratings with periodicity in the X directionare shown for the sake of example only. Different combinations of thesegratings with different biases and with different orientations can beprovided separately or as part of a target.

Starting with FIG. 7(a) a target 600 formed in two layers, labeled L1and L2, is shown. In the lowermost or bottom layer L1, a first structure(the lowermost or bottom structure), for example a grating, is formed byfeatures 602 and spaces 604 on a substrate 606. In layer L2 a secondstructure, for example a grating, is formed by features 608 and spaces610. (The cross-section is drawn such that the features 602, 608 (e.g.,lines) extend into the page.) The grating pattern repeats with a pitch Pin both layers. Features 602 and 608 may take the form of lines, dots,blocks and via holes. In the situation shown at (a), there is no overlaycontribution due to misalignment, e.g., no overlay error and no imposedbias, so that each feature 608 lies exactly over a feature 602 in thefirst structure.

At FIG. 7(b), the same target with a first known imposed bias +d isshown, such that the features 608 of the first structure are shifted bya distance d to the right, relative to the features of the secondstructure. The bias distance d might be a few nanometers in practice,for example 10 nm-20 nm, while the pitch P is for example in the range300-1000 nm, for example 500 nm or 600 nm. At FIG. 7(c) we see anotherfeature with a second known imposed bias −d, such that the features of608 are shifted to the left. Biased targets of this type shown at (a) to(c) are well known in the art, and used in the prior applicationsmentioned above.

FIG. 7(d) shows schematically a phenomenon of structural asymmetry, inthis case structural asymmetry in the first structure (bottom gratingasymmetry). The features in the gratings at (a) to (c), are shown asperfectly square-sided, when a real feature would have some slope on theside, and a certain roughness. Nevertheless they are intended to be atleast symmetrical in profile. The features 602 and/or spaces 604 at (d)in the first structure no longer have a symmetrical form at all, butrather have become distorted by processing steps. Thus, for example, abottom surface of each space has become tilted. Side wall angles of thefeatures and spaces have become asymmetrical also. As a result of this,the overall target asymmetry of a target will comprise an overlaycontribution independent of structural asymmetry (i.e., an overlaycontribution due to misalignment of the first structure and secondstructure; itself comprised of overlay error and any known imposed bias)and a structural contribution due to this structural asymmetry in thetarget.

When overlay is measured by the method of FIG. 6 using only two biasedgratings, the process-induced structural asymmetry cannot bedistinguished from the overlay contribution due to misalignment, andoverlay measurements (in particular to measure the undesired overlayerror) become unreliable as a result. Structural asymmetry in the firststructure (bottom grating) of a target is a common form of structuralasymmetry. It may originate, for example, in the substrate processingsteps such as chemical-mechanical polishing (CMP), performed after thefirst structure was originally formed.

In WO 2013/143814 A1, it is proposed to use of three or more componenttargets to measure overlay by a modified version of the method of FIG.6. Using three or more targets of the type shown in FIG. 7(a) to (c) areused to obtain overlay measurements that are to some extent correctedfor structural asymmetry in the target gratings, such as is caused bybottom grating asymmetry in a practical lithographic process. However,this method requires a new target design (e.g. different to thatillustrated in FIG. 4) and therefore a new reticle will be required.Furthermore, the target area is larger and therefore consumes moresubstrate area.

In FIG. 8 a curve 702 illustrates the relationship between overlay OVand intensity asymmetry A for an ‘ideal’ target having zero offset andno structural asymmetry within the individual gratings forming thetarget. Consequently, the target asymmetry of this ideal targetcomprises only an overlay contribution due to misalignment of the firststructure and second structure resultant from a known imposed bias andoverlay error OV_(E). This graph, and the graph of FIG. 9, is toillustrate the principles behind the disclosure only, and in each graph,the units of intensity asymmetry A and overlay OV are arbitrary.Examples of actual dimensions will be given further below.

In the ‘ideal’ situation of FIG. 8, the curve 702 indicates that theintensity asymmetry A has a non-linear periodic relationship (e.g.,sinusoidal relationship) with the overlay. The period P of thesinusoidal variation corresponds to the period or pitch P of thegratings, converted of course to an appropriate scale. The sinusoidalform is pure in this example, but can include harmonics in realcircumstances.

As mentioned above, biased gratings (having a known imposed overlaybias) can be used to measure overlay, rather than relying on a singlemeasurement. This bias has a known value defined in the patterningdevice (e.g. a reticle) from which it was made, that serves as anon-wafer calibration of the overlay corresponding to the measuredintensity asymmetry. In the drawing, the calculation is illustratedgraphically. In steps S1-S5, intensity asymmetry measurements A^(+d) andA^(−d) are obtained for targets having imposed biases +d an −drespectively (as shown in FIGS. 7 (b) and (c), for example). Fittingthese measurements to the sinusoidal curve gives points 704 and 706 asshown. Knowing the biases, the true overlay error OV_(E) can becalculated. The pitch P of the sinusoidal curve is known from the designof the target. The vertical scale of the curve 702 is not known to startwith, but is an unknown factor which can be referred to as a 1^(st)harmonic proportionality constant, K₁. This constant K₁ is a measure ofthe sensitivity of the intensity asymmetry measurements to the target.

In equation terms, the relationship between overlay error OV_(E) andintensity asymmetry A is assumed to be:

$\begin{matrix}{A_{\pm d} = {\frac{2\; \pi}{P}K_{1}{\sin \left( {{OV}_{E} \pm d} \right)}}} & (1)\end{matrix}$

where overlay is expressed on a scale such that the target pitch Pcorresponds to an angle 2π radians.

As overlay is very small, this relationship can be approximated to alinear relationship over the range of interest, using the assumptionsin(OV_(E)±d)=OV_(E)±d:

$\begin{matrix}{A_{\pm d} \approx {\frac{2\; \pi}{P}{K_{1}\left( {{OV}_{E} \pm d} \right)}}} & (2)\end{matrix}$

Using two measurements of targets with different, known biases (e.g. +dand −d) the overlay error OV_(E) can be calculated.

FIG. 9 shows a first effect of introducing structural asymmetry, forexample the bottom grating asymmetry illustrated in FIG. 7(d). The‘ideal’ sinusoidal curve 702 no longer applies. However, at leastapproximately, bottom grating asymmetry or other structural asymmetryhas the effect of adding an offset term to the intensity asymmetry A,which is relatively constant across all overlay values. The resultingcurve is shown as 712 in the diagram, with label K₀ indicating theoffset term due to structural asymmetry. Offset term K₀ is dependentupon a selected characteristic of the measurement radiation, such as thewavelength and polarization of the measurement radiation (the“measurement recipe”), and is sensitive to process variations. Inequation terms, the relationship used for calculation in step S6becomes:

$\begin{matrix}\begin{matrix}{A_{\pm d} = {K_{0} + {\frac{2\; \pi}{P}K_{1}{\sin \left( {{OV}_{E} \pm d} \right)}}}} \\{\approx {K_{0} + {\frac{2\; \pi}{P}{K_{1}\left( {{OV}_{E} \pm d} \right)}}}}\end{matrix} & (3)\end{matrix}$

the above approximation again being valid for the overlay range ofinterest.

Where there is structural asymmetry, the overlay model described byEquation (2) will provide overlay error values which are impacted by theoffset term K₀, and will be inaccurate as a consequence. The structuralasymmetry will also result in differences in measurements of the sametarget using different measurement recipes, when mapping the overlayerror, because the intensity shift described by the offset term iswavelength dependent. At present there is no method to remove theoverlay contribution due to structural asymmetry in a single measurementstep, thereby correcting the overlay error measurements. Therefore, athroughput penalty is incurred to correct for the offset term K₀, orslight changes in substrate processing will lead to overlay variation,thereby impacting the overlay control loop APC (Automatic ProcessControl) and the device yield.

It is proposed to measure the target asymmetry of a target, andtherefore overlay which does not neglect the effect of the structuralasymmetry, while allowing the use of current target designs such asthose illustrated in FIG. 4. This modelling may be performed as amodification to step S6 in the method illustrated in FIG. 6. The methodproposed can calculate overlay errors accurately using real substratemeasurement data, and which can determine the optimal or preferredcombination of targets and measurement recipes. No simulation orreconstruction is needed.

It is therefore proposed to add an array of lenses, which may be amicrolens array, into the pupil plane of the second measurement branchof the arrangement of FIG. 3(a). In an embodiment this may be at or nearthe focal plane of the output lens assembly 20 defined by its focallength f₂₀. This provides a number of advantages over the previousarrangements described. Each microlens of the microlens array creates anindividual image or sub-image from a localized section of the pupil.From the local sub-image, a local overlay calculation can be performed.The positions of the sub-images provide an aberration distribution overthe exit pupil. This aberration distribution allows correction to bemade for the aberration, and also allows focus to be determined. Thisdetermination of focus within the second measurement branch (that usedfor performing μDBO measurements) means that the first measurementbranch as depicted in FIG. 3(a) may be dispensed with.

The resultant plenoptic image can be thought of as a hybrid imagecomprising both image plane information (the sub-images) and thewavefront distortion information (from the relative positions of thesub-images). Additionally, the intensities of the sub-images allow apupil image to be calculated which is free from the influence ofstructural asymmetry of from product structure. Such a pupil image canbe constructed from the relative intensities of the sub-images comprisedwithin an image. The pupil image can be used for reconstructingstructural asymmetry and for performing CD metrology on small (finite)targets.

FIG. 10 shows a metrology apparatus suitable for performing overlaymeasurements. It is largely similar to the metrology apparatus shown inFIG. 3(a), but only comprising a single measurement branch, equivalentto the second measurement branch of FIG. 3(a), such that the sensor 23is in the image plane. The elements common with the metrology apparatusshown in FIG. 3(a) will not be described further.

A microlens array 60 is located between the pupil plane PP of theoptical system, and the image plane (sensor 23). The term ‘pupil plane’in this context refers to any plane in the optical system which is animage of the ‘Back focal plane’ BFP of the objective lens 16. The outputlens assembly 20 creates an image of the back focal plane BFP of theobjective lens 16. The microlens array 60 may comprise a 2 dimensionalarray of individual lenses (or microlenses). The microlens array 60 maybe such that it is located at the pupil plane PP (or more specificallyat the focal plane of the output lens 20). Alternatively, the microlensarray 60 may be located at a distance f_(ml) from both the pupil planePP and the image plane, where distance f_(ml) is the focal length ofeach microlens within the microlens array (assuming all microlenses inthe array are similar). In practice, it may be preferred to place themicrolens array at the exact focal plane of the output lens 20 in orderto minimize vignetting, although it may be more convenient to place themicrolens array forward of this, such that the transform relation isunencumbered with quadratic phase factors; for example in the locationshown in FIG. 10 at a distance from the pupil plane equal to the focallength f_(ml) of each microlens of the microlens array.

Each microlens produces a focused image on the sensor 23. The microlensarray 60 replaces the final lens 22 and any prisms (not shown) of themetrology apparatus shown in FIG. 3(a). In this configuration, theapparatus is similar to a Shack-Hartmann sensor. The radiation profileat exit pupil 64 is imaged by the microlens array 60 onto sensor 23,resulting in a plenoptic image 66 comprising sub-images 68 a, 68 b 70,each created by one microlens of the microlens array 60. Higher ordersub-images 68 a are formed from one of the first orders of scatteredradiation, higher order sub-images 68 b are formed from the other of thefirst orders of scattered radiation, and zeroth order sub-images 70 areformed from the zeroth order of scattered radiation. Higher ordersub-images 68 a and 68 b may also comprise images formed from ordershigher than the first order. The zeroth order sub-images 70 may beattenuated by an absorbing neutral density filter so that they arewithin the dynamic range of the sensor or a high dynamic range detectormay be used. Alternatively, the zeroth order may be blocked altogether.

A local overlay from each sub-image 68 a, 68 b can be calculated. Thiscan be done using known μDBO processing techniques such as those alreadydescribed (e.g., from intensity asymmetries from biased gratings).However, as will be described below, these local values may be combinedto obtain an overall overlay value. The combination may include aweighting of the local overlay values.

Where the imaging is ideal, with no aberrations (for example, defocusaberration or aberration introduced by the imaging system), thesub-images 68 a, 68 b, 70 will lie on a uniform 2D grid. This assumesthat the microlenses are arranged in a uniform 2D grid in the microlensarray 60. Should there be any aberration, this will manifest itself interms of a wavefront curvature (phase distribution offsets). Themicrolens array 60 allows measurement of this wavefront curvature; thiscan be done by measuring the deviation of the positions of thesub-images 68 a, 68 b, 70 relative to a uniform grid (grid distortion).From this grid distortion, it can be determined which part of the pupilhas the most distortion, and therefore which is most sensitive toaberration. Weighting can then be applied over the pupil depending onwhich parts of the pupil are the most process dependent. This can besimulated in advance from a sensor model when the type of target beingused is known. The most process dependent parts of the pupil can also beinferred from measurements, for example, by looking at parts of thepupil which have a relatively higher intensity.

This measured aberration can be further decomposed into aberrationresulting from defocus and aberration resulting from other sources (lensaberrations, other imaging aberrations etc.). This is because the griddistortion measured from plenoptic image 66 which results from non-focusaberrations tends to be static for different degrees of defocus, whilethe grid distortion due to defocus will vary with (de)focus. Bycorrecting out the grid distortions resultant from non-focusaberrations, correct focus can be inferred from the plenoptic image 66as that corresponding to there being no grid distortion in plenopticimage 66. Defocus of the plenoptic image in a first direction willresult in distortion from a grid in a first direction (e.g., radiallyoutwards with respect to the grid center). Similarly, defocus of theplenoptic image in a second direction will result in distortion from agrid in a second direction (e.g., radially inwards with respect to thegrid center).

The grid distortion which results from non-focus aberrations may becorrected for by performing initial calibration measurements; this maycomprise obtaining plenoptic images 66 at two or more set degrees ofdefocus, and calculating the static grid distortion in thesemeasurements. Such calibration also allows, where the plenoptic image isdefocussed, the degree of defocus to be measured.

While the above methodology is described mainly in terms of defocus, itmay be used for any aberration. Any aberration over the pupil can bedetected and an aberration free, angularly resolved measurement foroverlay can be obtained. Smoothly varying aberrations can be thought ofas piecewise linear. Importantly, it is observed that the aberrationsonly cause the sub-images 68 a, 68 b. 70 to deviate from the grid; theydo not change the local overlay measurements.

It is as a result of this ability to measure aberrations, and inparticular focus, that an additional measurement branch is notnecessarily required. The single measurement branch can be used toobtain overlay measurements of small compound targets (unresolvable in apupil image) by measuring intensity asymmetry in images of the targetsusing μDBO techniques. The single measurement branch can also be used todetermine whether the objective lens 16 is correctly focused on thesubstrate. The single measurement branch may further enable correctionof focus and/or non-focus aberrations obtained from pupil perturbationsderived from the measured grid distortions.

FIG. 11 is a flow diagram illustrating a method of performing overlaymeasurements, including calibration according to the methods disclosed.In a calibration stage, a weighting w(x,y) may be determined. In a firststep, measured data M(x,y) is obtained, comprising a raw plenoptic imagemade up of a number of sub-images arranged in a two dimensional array(x,y). From each of these sub-images, a local overlay value OVL(x,y) anda local measure of the grid distortion G(x,y) is determined. These stepsmay be repeated for a number of targets on a substrate, and ideally forall targets on a substrate. Furthermore, this may be done for aplurality of substrates (e.g., substrates corresponding to the samestack). As a result, the grid distortion G(x,y) may be determined forevery position on a substrate and for a plurality of substrates.Variance of the grid distortion G(x,y) dataset can be used to determinea weighting w(x,y) for the local overlay values OVL (x,y) in terms ofpupil position (x,y). The weighting w(x,y) may also be determined by aprior digital computation with knowledge of the target layout. By way ofexample, the weighting may be determined using a heatmap created fromthe variance in grid distortion G(x,y). This heatmap may show distortionin terms of pupil position. Therefore, the heatmap will indicate whichareas of the pupil display the greatest distortion, and are most processdependent and sensitive to aberrations. These areas may have a lowerweighting assigned thereto. In some cases the lower weighting may be azero weighting, such that local overlay values from the correspondingpupil area will be ignored when calculating an overall value. Theweighting may be binary, such that areas of low distortion (e.g., belowa threshold) are weighted at 100% and areas of high distortion (e.g.,above a threshold) are weighted at 0%. Or else the weighting may be ofgreater resolution, or continuous.

Additionally, the calibration stage may comprise determining machineconstants and process dependent parameters PDP from the grid distortionG(x,y).

Once the calibration stage is completed, the weighting can be used toobtain an overall overlay measurement value from a set of local overlayvalues obtained from a plenoptic image. This can be done for all theoverlay values OVL(x,y) obtained from the measurement data M(x,y) duringthe calibration phase. The weighting w(x,y) can also be applied to anysubsequent measurements. The measurement data M(x,y) will comprise aplenoptic image comprising plural (higher order) sub-images 68 a, 68 b,as before. Local overlay is calculated OVL(x,y) and the local overlayvalues combined into a single overall overlay value OVL in accordancewith the weighting w(x,y). In a specific embodiment, the combination maybe as follows:

$\begin{matrix}{{OVL} = \frac{\sum\; {\sum\; {{{ovl}\left( {x,y} \right)} \cdot {w\left( {x,y} \right)}}}}{\sum\; {\sum\; {w\left( {x,y} \right)}}}} & (4)\end{matrix}$

A further advantage of the apparatus of FIG. 10, is that it enables anoverlay value to be obtained from a single μDBO measurement, using onlya single measurement radiation wavelength and/or polarization. Thisallows for correction for structural asymmetry (e.g., bottom gratingasymmetry) within the measured target.

By filtering out the influence of the product structure, the overlay canbe measured independently with difference angles of incidence of themeasurement radiation on the target. Since true overlay is independentof the angle of incidence, while any offset introduced by structuralasymmetry within the target is dependent upon angle of incidence, it ispossible to separate the measured overlay into true overlay and thatcaused by structural asymmetry.

A known μDBO technique may comprise obtaining in a single measurement,images of a target, such as that illustrated in FIG. 5, for both +1 and−1 orders. The +1 and −1 orders (and optionally the zeroth order) may beseparated by prisms in the pupil plane, such that each image is separateon the sensor. As already described, the target may comprise one or morepairs of gratings having two different offsets, for example +d and −d.As a result, a single measurement of a target can yield sufficientinformation to cancel for K₁ in Equation (1) or (2), enabling theoverlay error OV_(E) to be determined assuming no structural asymmetry.However such a measurement will not yield sufficient information to alsocancel, or obtain a value for, K₀ when there is structural asymmetry (asthere always will be) and Equation (3) applies. Therefore, moremeasurements are required with, for example, measurement radiation of adifferent wavelength.

Using the metrology apparatus of FIG. 10, the resultant plenoptic image66 may comprise multiple pairs of individual higher order sub-images 68a, 68 b of the target (comprising gratings having two offsets), witheach pair of sub-images comprising a sub-image obtained using +1radiation and a sub-image obtained using −1 radiation. Each pair ofsub-images will correspond to a different angle of incidence.Consequently, a single measurement may yield sufficient information toobtain a value for the offset K₀ and to cancel for K₁, meaning thatEquation (3) can be solved for the overlay error OV_(E).

Clearly, there are a number of ways of solving for K₀ and determiningoverlay error OV_(E), using the general methods disclosed herein. In anembodiment, values of asymmetry measurements A^(+d) are plotted againstthe determined values of asymmetry measurements A^(−d), obtained fortargets having imposed biases +d an −d respectively. Each point on theplot corresponds to a pair of sub-images. The offset K₀ can be foundfrom the intersects with each axis, of a line fitted to these points,the line being described by:

$\begin{matrix}{\frac{A^{+ d} - K_{0}}{A^{- d} - K_{0}} = \frac{{OV}_{E} + d}{{OV}_{E} - d}} & (5)\end{matrix}$

The fact that the single plenoptic image provides sufficient points(i.e., two but preferably more) to plot a line enables this method to beused while also enabling the axes intersects to be found. By contrast,using a conventional image yields only a single point, and an assumptionneeds to be made that the line passes through the origin (i.e., there isno structural asymmetry). Alternatively, more than one measurement ismade, decreasing throughput.

Another advantage of the apparatus of FIG. 10 is that it enables betterasymmetry calibration as it provides robustness to the cross termbetween process asymmetry and sensor induced asymmetry. Because thepupil is resolved using the apparatus of FIG. 10, it is possible to alsodetermine asymmetry in the entrance pupil. This may be as a result of aphase aberration in the metrology system optics, which can be seen inthe resolved pupil. Consider a plane wave incident on the target. In ascalar approximation, the scalar field reflected by the target at a 2Dplane parallel to the target can be represented as

r(x,y)=a(x,y)e ^(iφ(x,y))  (6)

Where r(x, y) is the reflected field comprising of an amplitudedistribution, a(x, y) and a phase distribution y(x, y). If the target isa an overlay grating, the reflected field has an asymmetry manifested inthe amplitude term a(x, y). This scalar field propagates through thesensor's optics and is detected at a 2D intensity detector.

The scalar field at a pupil plane in the sensor is given by a scaledFourier transform of this field which is the action of a lens on aninput field in the scalar approximation. The field at the pupil planecan be described as

{tilde over (r)}(k _(x) ,k _(y))={tilde over (a)}(k _(x) ,k _(y))e^(iφ(k) ^(x) ^(,k) ^(y) ⁾  (7)

Where {tilde over (r)}(k_(x), k_(y)) represents the scaled Fouriertransform of the input scalar field.

Sensor aberrations largely manifest themselves as phase distributions inthe pupil plane. For example, a defocus can be described as a quadraticphase distribution over the pupil and a tilt can be described as alinear phase distribution in the corresponding direction.

In this case, the scalar field the aberrated pupil plane can bedescribed as

{tilde over (r)}′(k _(x) ,k _(y))={tilde over (a)}(k _(x) ,k _(y))e^(iφ(k) ^(x) ^(,k) ^(y) ⁾ e ^(iε(k) ^(x) ^(,k) ^(y) ⁾  (8)

Where e^(iε(k) ^(x) ^(,k) ^(y) ⁾ is the phase aberration.

If the overlay is measured in the pupil plane, i.e, if the 2D detectoris placed in the pupil plane, then the intensity recorded is

I _(p)(k _(x) ,k _(y))=|{tilde over (r)}′(k _(x) ,k _(y))|² =|ã(k _(x),k _(y))e ^(iφ(k) ^(x) ^(,k) ^(y) ⁾ e ^(iε(k) ^(x) ^(,k) ^(y) ⁾|² =|ã(k_(x) ,k _(y))|²  (9)

The pupil plane phase aberration does not impact the overlaymeasurement.

If however, the overlay is measured in the field plane. The distributionin the field plane is given by a scaled Fourier transform of the pupilplane distribution.

$\begin{matrix}{{a^{\prime}\left( {x,y} \right)} = {\int{\int_{- k}^{k}{{\overset{\sim}{a}\left( {k_{x},k_{y}} \right)}^{\; {\overset{\sim}{\phi}{({k_{x},k_{y}})}}}^{\; {ɛ{({k_{x},k_{y}})}}}^{\frac{2\; \pi}{\lambda \; f}{({{k_{x}x} + {k_{y}y}})}}\ {k_{x}}{k_{y}}}}}} & \; \\\begin{matrix}{{I_{f}\left( {x,y} \right)} = {{a^{\prime}\left( {x,y} \right)}}^{2}} \\{= {{\int{\int_{- k}^{k}{{\overset{\sim}{a}\left( {k_{x},k_{y}} \right)}^{\; {\overset{\sim}{\phi}{({k_{x},k_{y}})}}}^{\; {ɛ{({k_{x},k_{y}})}}}^{\frac{2\; \pi}{\lambda \; f}{({{k_{x}x} + {k_{y}y}})}}\ {k_{x}}{k_{y}}}}}}^{2}}\end{matrix} & (10)\end{matrix}$

The detected signal is the modulus square of the complex distribution.It can be seen that due to the Fourier transform operation, the sensorinduced aberration e^(iε(k) ^(x) ^(,k) ^(y) ⁾ couples completely intothe detected signal. Every detected pixel in I_(f)(x, y) experiencescontribution from the full pupil and the image is distorted by thesensor aberration.

This can also be seen via the convolution theorem. The Fourier transformof a multiplication can be expressed as a convolution of the individualFourier transforms.

$\begin{matrix}\begin{matrix}{{a^{\prime}\left( {x,y} \right)} = {\int{\int_{- k}^{k}{{\overset{\sim}{a}\left( {k_{x},k_{y}} \right)}^{\; {\overset{\sim}{\phi}{({k_{x},k_{y}})}}}^{\; {ɛ{({k_{x},k_{y}})}}}^{\frac{2\; \pi}{\lambda \; f}{({{k_{x}x} + {k_{y}y}})}}\ {k_{x}}{k_{y}}}}}} \\{= {{a\left( {x,y} \right)} \otimes {\int{\int_{- k}^{k}{^{\; {ɛ{({k_{x},k_{y}})}}}^{\frac{2\; \pi}{\lambda \; f}{({{k_{x}x} + {k_{y}y}})}}\ {k_{x}}{k_{y}}}}}}} \\{= {{a\left( {x,y} \right)} \otimes {\overset{\sim}{ɛ}\left( {x,y} \right)}}}\end{matrix} & (11)\end{matrix}$

Where {tilde over (ε)}(x, y) is the Fourier transform of the aberrationfunction. It can be seen that the complex distribution in the fieldplane is convolved by the Fourier transform of the pupil aberrationfunction and as a result is ‘blurred’ by the sensor aberrations. Forexample, if the sensor aberration is due to a defocus, the image gets adefocus blur. Thus the image in the field plane is distorted and is moreprone to focus errors than the image of the pupil plane.

The apparatus of FIG. 10 allows thick stacks to be imaged, due to theincreased depth of focus of each microlens. This is in contrast tograting images of thick stacks (e.g., 3D NAND) which show tilts whenmeasured off-axis (e.g., with prisms). These tilts depend on focus andtherefore stack thickness between the bottom and top grating.

The apparatus of FIG. 10 allows measurement of multi-target marks in asingle measurement. Multi-target marks comprise targets having gratingsin different layer combinations in a single mark; for example a mark maycomprise a target comprising gratings in layer 1 and layer 2 and atarget comprising gratings in layer 1 and layer 3. This allowsmeasurement of overlay between different layer combinations in a singlemeasurement. Each of these targets may have different pitches. The areaof the pupil which corresponds to a particular target depends on thepitch, and therefore the different targets may be resolved within thepupil. As a result, different sub-images will correspond to differenttargets, and therefore overlay for different layer combinations can beobtained in a single measurement.

For relatively larger pitches, higher diffracted orders are also presentin the exit pupil plane. The overlay of larger pitch structures may beobtained from a combination of many such sub-images, using sub-imagesappropriately selected from both the 1^(st) order and the higher orders.At the same time, a different set of sub-images may be chosen for theoverlay measurement of the target with a smaller pitch. For example, amulti-target overlay mark may have two different pitches, P1 and P2,such that P1 is a large pitch for which a higher order is detectedwithin the pupil in addition to the first order and P2 is a smallerpitch for which only the first order is detected at the edge of thepupil. The sub-images formed due to diffraction from the P1 target arisein two different regions of the pupil, whereas the sub-images from theP2 target are formed at the edge of the exit pupil. For an overlaymeasurement of the P1 target, a different set of images may be chosenand with different weighting than for an overlay measurement of the P2target. Consequently, using the plenoptic image, it becomes possible tomix and weight different sub-images for each grating in a multi-targetmark.

The apparatus of FIG. 10 may also be used for CD (critical dimension)reconstruction, and in particular CD reconstruction of small targets. CDreconstruction is normally done in the first measurement branch of theapparatus of FIG. 3(a) as it requires a diffraction spectrum (pupilimage) to be obtained and one or more profile parameters of a targetinferred from the pupil image using reconstruction techniques. Suchreconstruction techniques may comprise iteratively adjusting a candidateprofile parameter, modelling the resultant pupil image and comparingthis to the measured pupil image. This is repeated until the modelledand measured pupil images converge. Reconstruction techniques may alsocomprise (in addition or as an alternative) comparing the pupil image toa library of previous calculated pupil images corresponding to knownprofile parameters until a closest match is found. Because CDreconstruction requires a pupil image, imaging techniques have not beenpossible, and therefore it has not been possible to use small,overfilled targets which are smaller than the measurement spot. Ifoverfilled targets were to be used, product features in the vicinity ofthe target, within the measurement spot, would interfere with the pupilimage. Without an image, it is not possible to separate out the effectof the product features.

As already stated, a pupil image may be obtained from the plenopticimage.

However, because the pupil image has been obtained from an array of realimages (the plenoptic image), the plenoptic image can be inspected(e.g., using pattern recognition techniques) and the effect of theproduct structure can be filtered out. This can be done by identifyingthe sub-images which correspond to product structure and not using thesewhen constructing the pupil image. In this way, a 2D angle resolvedpupil image which is free of product structure can be obtained. Thispupil image can be used in reconstruction.

In an alternative embodiment to that described, a simultaneous phase andamplitude angle-resolved scatterometer may comprise, as alreadydescribed, a microlens array placed in the pupil plane with a sensor inits focal plane, thereby operating as a Shack-Hartmann type of wavefrontsensor. Therefore, compared to a scatterometer of a type similar to thefirst measurement branch of FIG. 3(a), which only relays the pupilinformation towards a CCD camera, the pupil information may also berelayed (e.g., via a beam splitter) to the wavefront sensor. Thewavefront sensor measures the local wave front gradient, which can betransformed into the wavefront by integration. In this type ofscatterometer both the phase and the amplitude information, imprintedonto the illumination light by the target, can be measuredsimultaneously. Using more information for reconstruction purposesshould make applications more robust to sensor imperfections andsuboptimal measurement conditions, effectively increasing theapplication's “process window”.

While the targets described above are metrology targets specificallydesigned and formed for the purposes of measurement, in otherembodiments, properties may be measured on targets which are functionalparts of devices formed on the substrate. Many devices have regular,grating-like structures. The terms ‘target grating’ and ‘target’ as usedherein do not require that the structure has been provided specificallyfor the measurement being performed. Further, pitch P of the metrologytargets is close to the resolution limit of the optical system of thescatterometer, but may be much larger than the dimension of typicalproduct features made by lithographic process in the target portions C.In practice the lines and/or spaces of the overlay gratings within thetargets may be made to include smaller structures similar in dimensionto the product features.

In association with the physical grating structures of the targets asrealized on substrates and patterning devices, an embodiment may includea computer program containing one or more sequences of machine-readableinstructions describing methods of measuring targets on a substrateand/or analyzing measurements to obtain information about a lithographicprocess. This computer program may be executed for example within unitPU in the apparatus of FIG. 3 and/or the control unit LACU of FIG. 2.There may also be provided a data storage medium (e.g., semiconductormemory, magnetic or optical disk) having such a computer program storedtherein. Where an existing metrology apparatus, for example of the typeshown in FIG. 3, is already in production and/or in use, the inventioncan be implemented by the provision of updated computer program productsfor causing a processor to perform the modified step S6 and so calculateoverlay error or other parameters with reduced sensitivity to structuralasymmetry.

The program may optionally be arranged to control the optical system,substrate support and the like to perform the steps S2-S5 formeasurement of asymmetry on a suitable plurality of targets.

While the embodiments disclosed above are described in terms ofdiffraction based overlay measurements (e.g., measurements made usingthe second measurement branch of the apparatus shown in FIG. 3(a)), inprinciple the same models can be used for pupil based overlaymeasurements (e.g., measurements made using the first measurement branchof the apparatus shown in FIG. 3(a)). Consequently, it should beappreciated that the concepts described herein are equally applicable todiffraction based overlay measurements and pupil based overlaymeasurements.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

Further embodiments according to the present invention are presented inbelow numbered clause:

1. A metrology apparatus for measuring a parameter of a lithographicprocess, the metrology apparatus comprising:

an optical system for measuring a target on a substrate by illuminatingthe target with measurement radiation and detecting the measurementradiation scattered by the target; and

an array of lenses, each lens being operable to focus the scatteredmeasurement radiation onto a sensor, said array of lenses therebyforming an image on the sensor such that said image comprises aplurality of sub-images, each sub-image being formed by a correspondinglens of said array of lenses.

2. A metrology apparatus according to clause 1 wherein said array oflenses comprises a plurality of similar lenses arranged in a regular 2Darray.

3. A metrology apparatus according to clause 1 or 2 wherein said sensoris located at the image plane defined by said array of lenses.

4. A metrology apparatus according to clause 1, 2 or 3 wherein saidarray of lenses is located at a pupil plane of the optical system.

5. A metrology apparatus according to clause 1, 2 or 3 wherein saidarray of lenses is located between a pupil plane of the optical systemand the image plane, and at a distance from the pupil plane equal to thefocal length of each lens of said array of lenses.

6. A metrology apparatus according to clause 4 or 5 wherein said opticalsystem comprises an output lens and an objective lens, said output lensbeing operable to form an image of a back focal plane of the objectivelens at said pupil plane.

7. A metrology apparatus according to any preceding clause wherein saidimage is a plenoptic image.

8. A metrology apparatus according to any preceding clause wherein atleast some of said sub-images are composed of higher orders of saidscattered measurement radiation.

9. A metrology apparatus according to any preceding clause beingoperable to measure deviation of the positions of said sub-imagesrelative to a regular 2D grid.

10. A metrology apparatus according to clause 9 wherein said metrologyapparatus is operable to:

measure intensity asymmetry in corresponding higher orders of thescattered measurement radiation from said sub-images; and

determine, from each of the measurements of intensity asymmetry, a localmeasurement of target asymmetry in a target being measured.

11. A metrology apparatus according to clause 10 being operable tocombine said local measurements of target asymmetry in the target beingmeasured, to obtain an overall measurement of target asymmetry.

12. A metrology apparatus according to clause 11 wherein said combiningof said local measurements of target asymmetry includes weighting saidlocal measurements of target asymmetry based on the deviation of thepositions of the corresponding sub-images relative to a regular 2D grid.

13. A metrology apparatus according to clause 12 wherein said localmeasurements of target asymmetry are given greater weighting the closerthe positions of its corresponding sub-image is to the regular 2D grid.

14. A metrology apparatus according to any of clauses 10 to 13 operableto:

obtain, in a single measurement step, a plurality of different valuesfor intensity asymmetry from different corresponding pairs of sub-imagesof a target comprising a first grating and a second grating, wherein thetarget asymmetry of the first grating comprises a structuralcontribution due to structural asymmetry, a first imposed targetasymmetry and an overlay error and the target asymmetry of the secondgrating comprises, the structural contribution due to structuralasymmetry, a second imposed target asymmetry and the overlay error; and

determine a value for the overlay error from said plurality of differentvalues for intensity asymmetry.

15. A metrology apparatus according to clause 14 wherein said structuralcontribution due to structural asymmetry results in an offset in therelationship between target asymmetry and intensity asymmetry describedby an offset term, and said determining a value for the overlay errorcomprises determining a value for said offset term.

16. A metrology apparatus according to 14 or 15 wherein said determininga value for the overlay error comprises plotting the intensity asymmetrymeasurements obtained from inspection of said first grating againstasymmetry measurements obtained from inspection of said second grating.

17. A metrology apparatus according to any of clauses 9 to 16 beingoperable to determine whether said optical system is correctly focusedbased on said deviation of the positions of the sub-images relative to aregular 2D grid.

18. A metrology apparatus according to clause 17 being operable to:

obtain an image for each of a plurality of focus settings;

determine the relationship between focus and said deviation of thepositions of the sub-images comprised with said images relative to aregular 2D grid; and

for subsequent images, determine focus from the deviation of thepositions of the sub-images comprised within each subsequent imagerelative to a regular 2D grid and said determined relationship.

19. A metrology apparatus according to clause 18 being further operableto determine a static component in said deviation of the positions ofthe sub-images relative to a regular 2D grid and correcting for thisstatic component.

20. A metrology apparatus according to any preceding clause operable tocalculate a pupil image from intensities of the sub-images.

21. A metrology apparatus according to clause 20 being operable to:

determine a phase distribution from said pupil image; and

determine aberrations in the optical system from said phasedistribution.

22. A metrology apparatus according to any preceding clause wherein saidtarget is smaller than a measurement field of the metrology apparatus,such that a measurement of the target also comprises influence fromstructures adjacent the target; said metrology apparatus being operableto:

identify which of said sub-images result from measurement radiationscattered from said structures adjacent the target and which of saidsub-images result from measurement radiation scattered from the target;and

reconstruct one or parameters of the target using only sub-images whichresult from measurement radiation scattered from the target.

23. A metrology apparatus according to any preceding clause wherein saidtarget comprises two or more gratings having different pitches; saidmetrology apparatus being operable to identify which sub-imagecorresponds to which target by its location within the image.

24. A metrology apparatus according to clause 23 wherein said two ormore gratings are composed of structures in different layercombinations, said metrology apparatus being operable to determine avalue for target asymmetry for each of said layer combinations in asingle measurement step.

25. A metrology apparatus according to clause 24 wherein the pitch of atleast one of said gratings is such that sub-images of said grating areformed on the sensor from different magnitude higher orders; and

said determination of a value for target asymmetry for each gratingcomprises using different sub-images with different weightings.

26. A lithographic system comprising:

a lithographic apparatus comprising:

an illumination optical system arranged to illuminate a pattern;

a projection optical system arranged to project an image of the patternonto a substrate; and

a metrology apparatus according to any preceding clause,

wherein the lithographic apparatus is arranged to use a determination oftarget asymmetry by the metrology apparatus in applying the pattern tofurther substrates.

27. A method of measuring a parameter of a lithographic process,comprising:

measuring a target on a substrate by illuminating the target withmeasurement radiation and detecting the measurement radiation scatteredby the target;

forming an image of the target, said image comprising a plurality ofsub-images, each sub-image being formed by a corresponding lens of anarray of lenses; and

measuring said parameter of a lithographic process from said image.

28. A method according to clause 27 wherein said array of lensescomprises a plurality of similar lenses arranged in a regular 2D array.

29. A method according to clause 27 or 28 wherein said image is formedat the image plane defined by said array of lenses.

30. A method according to any of clauses 27 to 29 wherein the array oflenses is located at a pupil plane of an optical system used forperforming the method.

31. A method according to any of clauses 27 to 29 wherein said array oflenses is located between a pupil plane of the optical system and theimage plane, and at a distance from the pupil plane equal to the focallength of each lens of said array of lenses.

32. A method according to any of clauses 27 to 31 wherein said image isa plenoptic image.

33. A method according to any of clauses 27 to 32 wherein saidsub-images are composed of higher orders of said scattered measurementradiation.

34. A method according to any of clauses 27 to 33 comprising measuringdeviation of the positions of said sub-images relative to a regular 2Dgrid.

35. A method according to clause 34 comprising:

measuring intensity asymmetry in corresponding higher orders of thescattered measurement radiation from said sub-images; and

determining, from each of the measurements of intensity asymmetry, alocal measurement of target asymmetry in a target being measured.

36. A method according to clause 35 comprising combining said localmeasurements of target asymmetry in the target being measured, to obtainan overall measurement of target asymmetry.

37. A method according to clause 36 wherein said combining of said localmeasurements of target asymmetry includes weighting said localmeasurements of target asymmetry based on the deviation of the positionsof the corresponding sub-images relative to the regular 2D grid.

38. A method according to clause 37 wherein said local measurements oftarget asymmetry are given greater weighting the closer the positions ofits corresponding sub-image is to the regular 2D grid.

39. A method according to any of clauses 35 to 38 comprising:

obtaining, in a single measurement step, a plurality of different valuesfor intensity asymmetry from different corresponding pairs of sub-imagesof a target comprising a first grating and a second grating, wherein thetarget asymmetry of the first grating comprises a structuralcontribution due to structural asymmetry, a first imposed targetasymmetry and an overlay error and the target asymmetry of the secondgrating comprises the structural contribution due to structuralasymmetry, a second imposed target asymmetry and the overlay error; and

determining a value for the overlay error from said plurality ofdifferent values for intensity asymmetry.

40. A method according to clause 39 wherein said structural contributiondue to structural asymmetry results in an offset in the relationshipbetween target asymmetry and intensity asymmetry described by an offsetterm, and said determining a value for the overlay error comprisesdetermining a value for said offset term.

41. A method according to 39 or 40 wherein said determining a value forthe overlay error comprises plotting the intensity asymmetrymeasurements obtained from inspection of said first grating againstasymmetry measurements obtained from inspection of said second grating.

42. A method according to any of clauses 34 to 41 comprising determiningwhether said optical system is correctly focused based on said deviationof the positions of the sub-images relative to a regular 2D grid.

43. A method according to clause 42 comprising:

obtaining an image for each of a plurality of focus settings;

determining the relationship between focus and said deviation of thepositions of the sub-images comprised within said images relative to aregular 2D grid; and

for subsequent images, determining focus from the deviation of thepositions of the sub-images relative to a regular 2D grid and saiddetermined relationship.

44. A method according to clause 43 comprising determining a staticcomponent in said deviation of the positions of the sub-images relativeto a regular 2D grid and correcting for this static component.

45. A method according to any of clauses 27 to 44 comprising calculatinga pupil image from intensities of the sub-images.

46. A method according to clause 45 comprising:

determining a phase distribution from said pupil image; and

determining aberrations in the optical system from said phasedistribution.

47. A method according to any of clauses 27 to 46 wherein said target issmaller than a measurement field of the metrology apparatus, such that ameasurement of the target also comprises influence from structuresadjacent the target; said metrology apparatus comprising:

identifying which of said sub-images result from measurement radiationscattered from said structures adjacent the target and which of saidsub-images result from measurement radiation scattered from the target;and

reconstructing one or parameters of the target using only sub-imageswhich result from measurement radiation scattered from the target.

48. A method according to any of clauses 27 to 47 wherein said targetcomprises two or more gratings having different pitches; said metrologyapparatus comprising identifying which sub-image corresponds to whichtarget by its location within the image.

49. A method according to clause 48 wherein said two or more gratingsare composed of structures in different layer combinations, saidmetrology apparatus comprising determining a value for target asymmetryfor each of said layer combinations in a single measurement step.

50. A method according to clause 49 wherein the pitch of at least one ofsaid gratings is such that sub-images of said grating are formed on thesensor from different magnitude higher orders; and

said determining of a value for target asymmetry for each gratingcomprises using different sub-images with different weightings.

51. A computer program comprising processor readable instructions which,when run on suitable processor controlled apparatus, cause the processorcontrolled apparatus to perform the method of any one of clauses 27 to50.

52. A computer program carrier comprising the computer program of clause51.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. A metrology apparatus for measuring a parameter of a lithographicprocess, the metrology apparatus comprising: an optical systemconfigured to measure a target on a substrate by illuminating the targetwith measurement radiation and to detect the measurement radiationscattered by the target; and an array of lenses, each lens configured tofocus the scattered measurement radiation onto a sensor, said array oflenses thereby forming an image on the sensor such that said imagecomprises a plurality of sub-images, each sub-image being formed by acorresponding lens of said array of lenses. 2.-8. (canceled)
 9. Themetrology apparatus of claim 1, further configured to measure deviationof the positions of said sub-images relative to a regular 2D grid.10.-19. (canceled)
 20. The metrology apparatus of claim 1, furtherconfigured to calculate a pupil image from intensities of thesub-images.
 21. (canceled)
 22. The metrology apparatus of claim 1,wherein said target is smaller than a measurement field of the metrologyapparatus, such that a measurement of the target also comprisesinfluence from structures adjacent the target; said metrology apparatusfurther configured to: identify which of said sub-images result frommeasurement radiation scattered from said structures adjacent the targetand which of said sub-images result from measurement radiation scatteredfrom the target; and reconstruct one or parameters of the target usingonly sub-images which result from measurement radiation scattered fromthe target.
 23. The metrology apparatus of claim 1, wherein said targetcomprises two or more gratings having different pitches; said metrologyapparatus further configured to identify which sub-image corresponds towhich target by its location within the image. 24.-25. (canceled)
 26. Alithographic system comprising: a lithographic apparatus comprising: anillumination optical system arranged to illuminate a pattern; and aprojection optical system arranged to project an image of the patternonto a substrate; and a metrology apparatus comprising: an opticalsystem configured to measure a target on a substrate by illuminating thetarget with measurement radiation and to detect the measurementradiation scattered by the target; an array of lenses, each lensconfigured to focus the scattered measurement radiation onto a sensor,said array of lenses thereby forming an image on the sensor such thatsaid image comprises a plurality of sub-images, each sub-image beingformed by a corresponding lens of said array of lenses, wherein thelithographic apparatus is arranged to use a determination of targetasymmetry by the metrology apparatus in applying the pattern to furthersubstrates.
 27. A method of measuring a parameter of a lithographicprocess, comprising: illuminating a target on a substrate withmeasurement radiation; detecting the measurement radiation scattered bythe target; forming an image of the target, said image comprising aplurality of sub-images, each sub-image being formed by a correspondinglens of an array of lenses; and measuring said parameter of alithographic process from said image.
 28. The method of claim 27,wherein said array of lenses is configured as a plurality of similarlenses arranged in a regular 2D array.
 29. The method of claim 27,wherein said image is formed at the image plane defined by said array oflenses.
 30. The method of claim 27, wherein the array of lenses islocated at a pupil plane of an optical system used for performing themethod. 31.-33. (canceled)
 34. The method of claim 27, comprisingmeasuring deviation of the positions of said sub-images relative to aregular 2D grid.
 35. The A method of claim 34, further comprising:measuring intensity asymmetry in corresponding higher orders of thescattered measurement radiation from said sub-images; and determining,from each of the measurements of intensity asymmetry, a localmeasurement of target asymmetry in a target being measured. 36.-38.(canceled)
 39. The method of claim 35, further comprising: obtaining, ina single measurement step, a plurality of different values for intensityasymmetry from different corresponding pairs of sub-images of a targetcomprising a first grating and a second grating, wherein the targetasymmetry of the first grating comprises a structural contribution dueto structural asymmetry, a first imposed target asymmetry and an overlayerror and the target asymmetry of the second grating comprises thestructural contribution due to structural asymmetry, a second imposedtarget asymmetry and the overlay error; and determining a value for theoverlay error from said plurality of different values for intensityasymmetry. 40.-41. (canceled)
 42. The method of claim 34, furthercomprising determining whether said optical system is correctly focusedbased on said deviation of the positions of the sub-images relative to aregular 2D grid. 43.-44. (canceled)
 45. The method of claim 27, furthercomprising calculating a pupil image from intensities of the sub-images.46. The method of claim 45, further comprising: determining a phasedistribution from said pupil image; and determining aberrations in theoptical system from said phase distribution.
 47. The method of claim 27,wherein said target is smaller than a measurement field of the metrologyapparatus, such that a measurement of the target also comprisesinfluence from structures adjacent the target; further comprising:identifying which of said sub-images result from measurement radiationscattered from said structures adjacent the target and which of saidsub-images result from measurement radiation scattered from the target;and reconstructing one or parameters of the target using only sub-imageswhich result from measurement radiation scattered from the target. 48.The method of claim 27, wherein said target comprises two or moregratings having different pitches; further comprising identifying whichsub-image corresponds to which target by its location within the image.49.-50. (canceled)
 51. A computer program comprising processor readableinstructions which, when run on suitable processor controlled apparatus,cause the processor controlled apparatus to perform the methodcomprising: illuminating a target on a substrate with measurementradiation; detecting the measurement radiation scattered by the target;forming an image of the target, said image comprising a plurality ofsub-images, each sub-image being formed by a corresponding lens of anarray of lenses; and measuring a parameter of a lithographic processfrom said image.
 52. (canceled)